1. Field of the invention
The present invention relates generally to a control system for an automatic transmission of an automotive vehicle, which transmission having a fluid drive device with a lock-up clutch and being associated with an engine of the vehicle capable of controlling an amount of fuel supplied apart from an accelerator pedal stepping degree. More particularly, the present invention relates to an improved control system for the lock-up clutch of the fluid drive device, which system being capable of realizing lacking-up of the fluid drive device under an accelerator-off state.
2. Description of the Related Art
In a known automotive vehicle mounted with an automatic transmission having a fluid drive device (e.g. torque converter) with a lock-up clutch for locking-up the fluid drive device by mechanically engaging between input and output elements of the fluid drive device with a lock-up operation of the lock-up clutch, a lock-up area for the lock-up operation of the lock-up clutch is determined in relation to a vehicle speed and a throttle valve opening degree, so as to obtain an improved fuel consumption ratio of an engine of the vehicle and a large braking force from the engine.
On the other hand, in a known automotive vehicle mounted with an engine having a fuel-cut device, a fuel-cut operation for cutting fuel-supply for the engine is performed by the fuel-cut device after the vehicle is in an accelerator-off state, i.e. an accelerator pedal being fully released (causing fully closed state of a throttle valve), in accordance with an engine speed, so as to improve a fuel consumption ratio of the engine. The fuel-cut operation is continued up to a moment when the engine speed lowers to a certain value or less because an engine stall occurs if the engine speed excessively lowers due to the fuel-cut operation, thereafter the fuel-supply for the engine is restarted.
In case of a known automotive vehicle mounted with both a fluid drive device with a lock-up clutch and an engine with a fuel-cut device, a fuel consumption ratio is further improved by jointly using a fuel-cut operation and a lock-up operation. In such type of automotive vehicle under a high vehicle speed and a large oil discharge of an oil pump, a coasting lock-up operation wherein the fuel-cut and lock-up operations are simultaneously performed is carried out. Moreover, if a lock-up operation is changed from an usual lock-up operation with no fuel-cut operation to the coasting lock-up operation with a fuel-cut operation causing lowering of a vehicle speed, the coasting lock-up operation is continued while coordinates of the vehicle speed and a throttle valve opening degree are still in the lock-up area.
For further improving the fuel consumption ratio, it is desirable to perform the coasting lock-up operation for a low vehicle speed. Particularly, when up-shifting is performed due to an accelerator pedal being in the accelerator-off state, it is desirable to perform a lock-up operation. However, in the known automotive vehicle in low vehicle speed, changing from non-locking-up state to a coasting locking-up state cannot be performed because an oil discharge amount of an oil pump is less than that in high vehicle speed and thereby an influence of a centrifugal oil pressure increases as described later. Moreover, since the coasting lock-up operation cannot be performed in a low vehicle speed, the engine speed may decrease to a speed equal to or lower than a speed in which a fuel-cut operation can be performed, and thereby even a fuel-cut operation cannot be performed in some cases.
Therefore, even if a lock-up operation is tried in accordance with a lock-up command under such an accelerator-off state, the lock-up operation is hardly made or fails due to an influence of the centrifugal oil pressure, so that a lock-up operation under the accelerator-off state (a coasting lock-up operation) cannot be realized. Thus, in a known control system for a lock-up clutch of a fluid drive device, the lock-up operation of the lock-up clutch is inhibited under the accelerator-off state.
An influence of the centrifugal oil pressure is described below by referring to FIG. 3c. In the automotive vehicle mounted with both a fluid drive device with a lock-up clutch and an engine with a fuel-cut device, at a moment when a lock-up solenoid for operating the lock-up clutch is changed from "off" to "on" depending on a lock-up command generated after the vehicle became in the accelerator-off state, an engine speed Ne has become lower than a turbine speed Nt of the fluid drive device (i.e. Ne&lt;Nt) due to a fuel-cut operation, contrarily to the relation between these speeds at the moment when the vehicle became in the accelerator-off state. In this state wherein Ne&lt;Nt, an oil pressure Pa in an applying-chamber of the fluid drive device is larger than an oil pressure Pr in a releasing-chamber of the fluid drive device (i.e. Pr&gt;Pa) taking into account an action of the centrifugal oil pressure because the centrifugal oil pressure generated in the applying chamber is proportional to Ne2 while the centrifugal oil pressure generated in the releasing chamber is proportional to {(Ne+Nt)/2}2. Thus, a force for preventing a lock-up operation of the lock-up clutch is generated due to the differential pressure (Pr-Pa), causing the above mentioned problem.